The development of optical networking, and technologies for wavelength division multiplexing (WDM) of data onto optical fibers, has provided a high capacity communication network backbone that is capable of supporting high bit rate applications. However, the delivery of data to users at rates required by those high bit rate applications is generally not supported. This is referred to as the “last mile problem”. The last mile problem is directed to providing efficiently upgradeable high bandwidth service to residential and small commercial users at a price that makes the services appealing to a wide audience.
Some access networks developed to solve the last mile problem include passive optical network (PON) equipment. The idea is that optical fiber is sparingly used to interconnect a plurality of optical network units (ONUs), each of which supply the data services to a single or a respective subset of subscribers. Further, all-optical or “passive” network equipment, i.e. optical switching, coupling and handling equipment that does not use electronic or other power-consuming devices, is used in access networks because of the cost savings in terms of power consumption, and maintenance.
Today's PON access networks permit data to be transmitted by ONU customer premise equipment over a limited span of optical fiber, to an Optical Line Terminal (OLT), (which is generally a part of a service provider central office (CO)). Access networks typically use two optical wavelength carriers, one for distributing data from the OLTs to the ONUs, and one for returning data from the ONUs to the OLT. Often, the same optical fiber is used for transmission in both directions. In the distribution direction, the OLT can encrypt and address the data so that only an intended one of the ONUs can receive and decrypt it. In the return direction, a mechanism for sharing the medium that reduces a probability of collision is required. Time division multiplexing (TDM) is usually used for this purpose. As will be recognized by those skilled in the art, TDM is a well known technique for time sharing that divides the return wavelength channel into a plurality of timeslots. The OLT allocates the timeslots to respective ONUs in a manner known in the art.
Because of the shared use of the optical fiber, and the absence of electrical signal reception equipment in the access network, a problem can arise with ONU transmission equipment in such a configuration. If one ONU laser lapses into an error state in which it is stuck “on”, the optical output of the faulty laser “collides” with transmissions of other ONUs entitled to access to the optical fiber. This results in a complete blockage of data transmission to the OLT. If an ONU laser is stuck in the “on” state, the subscriber network effectively becomes a unidirectional distribution network until the defective ONU is identified and repaired or replaced, because the return data channel is effectively taken down. It has been found that lasers used in ONUs sometimes get “stuck” in an active or “on” state. Since all of the ONUs share the return data channel wavelength, and an ONU laser can lapse into the error state at any time, there is no way to remotely identify the faulty ONU.
Consequently service technicians have to inspect each of the ONUs in the access network until the defective ONU is identified. Throughout this time, the return data channel remains unusable, resulting in inconvenienced customers. Rapid identification of faulty ONUs to permit rapid service is therefore important for maintaining subscriber satisfaction. There therefore remains a need for a method and system for rapidly identifying a faulty ONU in an access network.